1. Field of the Invention
The present invention relates to a spread illuminating apparatus, and more particularly to a spread illuminating apparatus used as an illuminating means for a liquid crystal display device.
2. Description of the Related Art
A demand for a liquid crystal display device operating in a low power consumption is increasing mainly for personal computers or portable telephones due to its low profile and light weight. Since a liquid crystal of the liquid crystal display device does not emit light by itself unlike a light emitting element such as a cathode-ray tube, a separate illuminating means is required when using the device in dark places. To satisfy the recent demand for downsizing and energy saving on electronic products, a spread illuminating apparatus of side light type (light conductive plate type) is used as an illuminating means for the liquid crystal display device.
FIG. 5 is a schematic view of a spread illuminating apparatus of side light type. In general, a spread illuminating apparatus 1 is constituted in such a manner that a bar-like light source 4 is disposed close to an end surface 3 of a transparent substrate 2 made of a light-transmissible material. The transparent substrate 2 formed into a plate-like rectangle has a light reflection pattern 17 on an upper surface thereof. The light reflection pattern 17 is composed of a plurality of grooves 18 substantially triangular in section and flat portions 19 adjacent to the grooves 18, both formed parallel to the end surface 3.
In order to achieve a uniform radiation on a surface of the transparent substrate 2 irrespective of the distance from the light source 4, the grooves 18 of the light reflection pattern 17 are disposed with different intervals from place to place. Specifically, the ratio of an occupied area of the grooves 18 to an occupied area of the flat portions 19 is set to gradually increase in proportion to the increase in distance from the end surface 3 of the transparent substrate 2. The above grooves 18 are not recognizable when viewing the screen because of their very fine constitution.
The light source 4 is generally composed of a bar-like light conductive member 7 made of a transparent material and a spot-like light source 9 disposed facing to an end of the light conductive member 7, and disposed close to the end surface 3 of the transparent substrate 2. And, the light conductive member 7 is provided with an optical path conversion means 11. The optical path conversion means 11 is formed at a surface 14 opposite to a surface 13 facing the end surface 3 of the transparent substrate 2 in such a manner, for example, that grooves triangular in section as light scattering portions 15 are provided so as to run in a thickness direction of the light conductive member 7, whereby light emitted from the spot-like light source 9 uniformly enters the end surface 3 of the transparent substrate 2.
In order for light to be uniformly emitted from the light conductive member 7, the triangular grooves are desirably formed such that the depth increases gradually in proportion to the increase in distance from the spot-like light source 9. And, for effectively using light traveling in the light conductive member 7, it is preferable to provide a light reflection member 12 substantially U-shaped in section so as to cover longitudinal peripheral surfaces except the surface 13 facing the end surface 3 of the transparent substrate 2 and the vicinity of the end surface 3 of the transparent substrate 2 thereby preventing light from leaking from the light conductive member 7.
In the optical path conversion means of the light conductive member 7, in case the light scattering portions 15 and flat portions 16 are alternately arranged, only the light scattering portions 15 function substantially to reflect light into the transparent substrate 2 as shown in FIG. 6. Consequently, on a microscopic level, the optical path conversion means emits light with a plurality of luminous and dark portions thus illuminating the transparent substrate 2 with a plurality of light beams L although visually not recognizable. As a solution to the above, the present inventors came up with an idea that a diffusion plate 20 is provided between the light conductive member 7 and the transparent substrate 2 as shown in FIGS. 7 and 8, and Japanese Patent Application Laid-open No. 2000-231814 discloses the detail thereof.
However, in the conventional structure shown in FIGS. 7 and 8, the idea must have the diffusion plate 20 provided as an additional component, which is small in thickness and width thereby giving difficulties in handling and assembling, and making a cost increase inevitable. Further, the fact that light passes through the diffusion plate 20 causes light transmissivity to decrease and the light to diffuse, thereby lowering luminance and contrast.
The present invention has been made in the light of the above, and its object is to make linear light beams of a spread illuminating apparatus more uniform, thereby making its illumination spread more uniformly and at the same time improving luminance and contrast without increasing the number of components.
In order to solve the above problems, according to a first aspect of the present invention, in a spread illuminating apparatus of side light type which comprises a light source composed of a bar-like light conductive member made of a transparent material and a spot-like light source and disposed close to an end surface of a transparent substrate made of a light-transmissible material, at least one surface of the bar-like light conductive member has an optical path conversion means comprising a plurality of light scattering portions which are inclined in a longitudinal direction of the light conductive member.
The light scattering portions inclined in the longitudinal direction of the light conductive member expands an area range per light scattering portion in the longitudinal direction of the light conductive member, which expands an irradiation range per beam of light that is reflected at the light scattering portions and enters the transparent substrate, thereby narrowing a range per dark portion in the light conductive member even on a microscopic level.
According to a second aspect of the present invention, in the spread illuminating apparatus, the light scattering portions are formed in such a manner that the irradiation ranges of adjacent light scattering portions overlap partially with each other in the longitudinal direction of the light conductive member. In accordance with the structure, the irradiation ranges of light reflected at the respective adjacent light scattering portions overlap partially with each other, whereby light beams coming from the light conductive member and entering the transparent substrate can constitute uninterrupted illumination in the longitudinal direction of the light conductive member.
According to a third aspect of the present invention, in the spread illuminating apparatus, an inclination angle xcex8 of the light scattering portions is set: xcex8xe2x89xa7tanxe2x88x921 (p/t) where t is the thickness of the light conductive member and p is the pitch of the light scattering portions.
When the inclination angle xcex8 is set to satisfy the condition, the adjacent light scattering portions can overlap with each other in the longitudinal direction of the light conductive member. Thus, the ranges of light beams reflected at the adjacent light scattering portions overlap with each other, whereby light coming from the light conductive member can enter the transparent substrate in an uninterrupted manner with regard to the longitudinal direction of the light conductive member.
According to a fourth aspect of the present invention, in the spread illuminating apparatus, the light scattering portion is formed to be a groove. With the above formation of the light scattering portion, light is reflected so as to be guided from the light conductive member into the transparent substrate.
According to a fifth aspect of the present invention, in the spread illuminating apparatus, the optical path conversion means is composed of a plurality of grooves and a plurality of flat portions adjacent thereto. In accordance with the above structure, the grooves function as the light scattering portions to reflect and guide light from the light conductive member into the transparent substrate.
According to a sixth aspect of the present invention, in the spread illuminating apparatus, the grooves are shaped triangular in section, and according to a seventh aspect of the present invention, the grooves are shaped trapezoidal in section. A plurality of grooves with the above shapes function as the light scattering portions to reflect and guide light from the light conductive member into the transparent substrate.
According to an eighth aspect of the present invention, in the spread illuminating apparatus, the bar-like light conductive member may be formed in such a manner that either or both of the thickness and width thereof decrease from an end having the spot-like light source toward an opposite end so that the light conductive member has its minimum cross section at the opposite end. By reducing light leaking from the opposite end with the above construction, light can be uniformly radiated from the whole of the light conductive member irrespective of the distance from the spot-like light source thereby efficiently guiding the light into the transparent substrate.